Significant response to pralsetinib in a medullary thyroid cancer harboring double RET variants of unknown significance

Medullary thyroid carcinoma (MTC) is a rare but aggressive thyroid tumor, with 25% of hereditary and 75% of sporadic forms. RET mutations are found in 98% of hereditary MTC and in 55% of sporadic MTC (1). The most frequent somatic RET mutation occurs in codon M918, reported in up to 90% of RET-positive MTC cases (2). Selpercatinib and pralsetinib, tyrosine-kinase inhibitors with high specificity for RET protein, recently obtained FDA approval for the treatment of lung and thyroid cancers with RET gene mutations or fusions (3, 4). In MTC patients, phase I/II studies with RET inhibitors reported overall response rates of 73% and phase III studies are ongoing (5, 6).